Such seismic sources and audio generators respectively for deep water survey have been known and are used extensively e.g. to determine the profile of the sub-bottom terrain in underwater surveying. The present invention is concerned with an improved underwater seismic source of the kind which employs a piston plate which is moved and preferably repelled outwardly by an electromagnetic pulse produced by an electromagnetic coil. Optionally, the electromagnetic coil is driven in such a way that eddy currents are induced in the plate resulting in opposing electromagnetic fields moving the plate away from the coil and producing a pressure pulse in the water. The echoes produced by this pulse are then recorded e.g. by a hydrophone and electronic recorder. The pattern and variation in the echoes can be used to assess the strength and stratification of the underlying seabed. For marine seismic surveys (at sea, on rivers and on lakes), such acoustic pulses are generated under water resulting in seismic signals traveling down through the seabed and being reflected before the echoes are recorded.
In deep water surveying and exploration, when operating e.g. at depths in excess of 3,000 meters, the hydrostatic pressure is very high. Seismic sources (transmitter) operating at these depths are exposed to pressure of around 300 bar. Due to the high hydrostatic pressure, low frequency acoustic energy is difficult to generate by conventional methods. Specifically, there is a poor generation of low frequencies from the seismic source into the water. However, such low frequencies provide improved penetration in certain types of geology of the sea bottom. Therefore, these frequencies are necessary to provide the desired geological information.
A seismic source for deep water survey is e.g. disclosed in U.S. Pat. No. 3,993,973 comprising a structure having a coil and a piston plate arranged within a housing of a seismic source. By energizing the coil the plate can be repelled producing an acoustic signal. The coil and plate are arranged in such a way that as the device is raised or lowered in depth in the water, the ambient pressures on the front and rear faces of the plate would vary identically, resulting in the pressure pulse produced by the plate remaining constant over varying depths of submergence of the device. Nevertheless, the disclosed seismic source has a maximum operating depth, which occurs when a rear diaphragm enclosing a gas space is pressed against a fillet. Any further increase in depth will result in a change in the ambient pressure between the front and rear faces of the piston, and undesirable changes to the acoustic pulse signature.
A similar embodiment is disclosed by U.S. Pat. No. 4,875,199 also suffering from negative pressure influences acting on the seismic source over various water depths. U.S. Pat. No. 4,875,199 propose a deep water transient sound generator trying to overcome the deficiencies of U.S. Pat. No. 3,993,973, and in detail trying to eliminate the pressure differences between a front face and a rear face of the piston plate. In use, the piston plate according to U.S. Pat. No. 4,875,199 is driven out into the surrounding water creating the desired acoustical pulse, however, still suffering from signal losses in low frequency ranges, namely 400 to 2,000 Hz.
It is therefore an object of the present invention to provide a depth-independent seismic source that has improved transfer of low frequency acoustic energy into the surrounding marine environment independent of a variety of water depths and especially deep water depths over 500 m the source is operated.
This object is solved by a pressure tolerant seismic source according to claim 1.